The present invention relates to an optical modulator of clock modulation type.
In accordance with recent increases in data transmission rates, development of an optical modulator for modulating a data signal into an optical signal from an electric signal has been performed vigorously in the field of long-distance optical communications systems, such as in a submarine optical communication system. In the field of long distance optical communications systems such as in submarine optical communication, attention has been paid to a transmission scheme using an RZ (return-to-zero) signal as an optical signal to be transmitted rather than to another transmission scheme using an NRZ (non-return-to-zero) signal, because the transmission scheme using an RZ signal has a wide range of tolerance of dispersion and an improved receiving sensitivity characteristic.
When RZ optical modulation is effected through use of an external optical modulator in order to transmit the previously-described optical RZ signal, there is a necessity for a driver circuit which is higher in frequency than a driver circuit for effecting NRZ optical modulation. Manufacture of a driver circuit for subjecting data of 40 Gb/s to RZ modulation using a current semiconductor element is difficult.